Where does the water on Earth come from?28 May 2019 | Facts & Curiosities
It’s 2019, and it’s hard to imagine a question that can’t be answered with a quick Google search or a visit to the University of YouTube. It might therefore surprise you to learn that the finest minds in science can’t yet agree on the origins of one of the most ordinary substances on Earth – good old H20.
Water covers two-thirds of our planet, but much of it is still a mystery to us. We’ve mapped around 90% of our land, but less than 5% of our water. There are parts of space that we know more about than the bottom of the ocean. Things get even murkier if you include the solid water captured in glaciers and the polar ice caps – which in the past few years alone have spat out frozen woolly mammoths, sabretooth tigers, 30,000 year old mega viruses, blood-red underground rivers and even a new contender for the grandest canyon on Earth. As this process continues, we’ll no doubt discover new forms of life – and probably very old forms of life as well – assuming it doesn’t cause a global climate catastrophe first, that is.
There’s so much we don’t know about the watery world below, and it’s not exactly easy to find out more either. “On land, life exists in a thin layer that begins a few feet below the surface, and extends up into the tops of the trees,” says NASA Oceanographer Gene Feldman. “But in the water, we have life from the surface right down to at least seven miles below – where the pressure is the same as sitting under a pile of fifty jumbo jets. 99% of the habitats on Earth are aquatic, but we’ve seen only a tiny fraction of them so far. Every time we turn over a rock there’s something new – that’s why oceanography is so fascinating, but also so challenging”.
To be fair to humanity, we can kind of give ourselves a break for not yet charting the crushing depths of the Mariana Trench or every snowflake in the frozen wastelands of Siberia. But somewhat harder to understand is the idea that nobody even really knows where the water on Earth came from, or whether it’s always been here. Let’s take a look at the top theories, and you can decide for yourself which one “wets” your appetite. Grab your glasses and get ready for Top of the Drops, our watery rundown of… okay we’ll stop. But we did find some of this pretty fascinating!
Theory #1 – The Native Water hypothesis
Scooping the awards for both “most obvious” and “surprisingly controversial” is the idea that water has always been here on Earth, since it was first formed from the cosmic dust of the cooling universe. Water is made from oxygen and hydrogen, which are fairly abundant among the stars – so it’s perfectly plausible that these building blocks were present in the primordial soup that formed our world.
Researchers at the University of Hawaii, just off the beach in downtown Honolulu (which we’ve got to say, sounds like a pretty tricky place to concentrate on your lab work), are some of the leading proponents of this theory. They claim that the discovery tiny droplets of water inside glass crystals, inside other rocks harvested from the Earth’s mantle, proves there are masses of water stored deep inside the planet as well as all over its surface. This would mean that Earth formed “as a wet planet, with water already on its surface, since the figurative day dot”. It would also mean that any old rocky planet out there was probably watery at one point – which would likely mean a sharp uptick in our odds of one day find life like our own among the stars.
So far so good. So why is native water controversial?
Enter theory #2 – The Exotic Water hypothesis
Exotic Water sounds like something you’d pay far too much for in a pretentious nightclub. In this case, though, it refers to the idea that water might have arrived here from elsewhere in the solar system, millions of years after the Earth was formed.
The University of Hawaii’s research made waves because, for many years, we kind of thought we had it all figured out. Scientists believed that our planet spent its first couple of million years a molten mass of superheated rock, so any water by-products would have boiled away in the blink of an eye. It was thought that water must therefore have arrived here some time after, most likely in the form of icy comets and asteroids from the edges of our solar system and beyond.
The theory goes that over a timeline of millions of years, countless water-rich projectiles collided with our Earth at speeds of more than a million miles an hour (!), depositing their liquid payloads and eventually forming an atmosphere. Scientists had been able to prove that water on Earth shares the same chemical signature as water found in asteroids, down to the ratio of specific hydrogen isotopes. They had modelled how water could survive the enormous force and heat of an asteroid impact to remain on the newly-formed Earth. It all seemed to fit, so for many years we were convinced that water was extra-terrestrial in origin and indeed had probably travelled millions of light years to make Earth its home (cue X-Files theme tune).
Fun fact: there’s also an idea (that we touched on in a previous blog post) that life itself might be alien in nature, arriving here on the same asteroids as exotic water. It sounds more like the plot of a Hollywood blockbuster than a discussion serious scientists might have, but there’s a team of geniuses at MIT and NASA who are fairly convinced that’s how it all started. The truth is out there!
Theory #3 – The Wild Card
The new kid on the block is Nebula Water, or “Chondritic Inheritance Plus Nebular Ingassing” to its friends. This school of thought came into being only around 2018, and it kind of reads like the native and exotic water camps settled their differences, buried the hatchet and decided to have a baby together instead.
This theory contends that billions of years ago, fierce solar winds from our young sun scattered enormous amounts of hydrogen throughout the newly born solar system and out across the vast emptiness of space. Historically, we believed that most of that hydrogen ended up at the outskirts of the solar system, where it mixed with cosmic dust and eventually coalesced into gas giants like Jupiter, Saturn, Uranus and Neptune. Hydrogen that missed out on becoming part of the gas-giant supergroup amalgamated into icy asteroids instead, which is why we find so many of them in the vast asteroid belts near Jupiter and Neptune. This part is all gold for the Exotic Water From Outer Space camp.
The Nebula Water hypothesis, however, posits that the sun gifted watery goodness to everyone at the party and not just the gas giants in the back. Closer to our celestial body, it mixed with the embryotic magma of planets like Earth and did a dance called “isotopic fractionation” instead – locking the hydrogen deep inside and sewing the seeds for native water as well. According to team Nebula Water, it was millions of years of isotopic fractionation as well as visits from water-rich asteroids that eventually gifted the Earth its atmosphere. Isn’t that nice? Now we can all be friends.
Why does it matter?
The biggest implications are to do with the habitability of other worlds. It’s no secret that our own planet is already groaning under the weight of 7 billion humans, and some people – including the late, great Steven Hawking – believe we must become an interplanetary species, or we might not make it. Some very intelligent people are already making a very serious case for looking very, very hard at which lump of rock might become humanity’s next home – and that’s not a decision we want to take lightly.
If Native Water is correct, then there’s some good news – we may be looking at many more suitable candidates than we first imagined. Seemingly barren planets could hold great troves of precious, life-giving water beneath their rocky facades… if we can figure out how to unlock it.
If Exotic Water is correct, then we know to search for asteroid belts and gas giants as markers of habitable zones among the stars. If you believe the guys at MIT and NASA, we should also explore with light footsteps – because some of our potential new homes might already be occupied.
If Nebula Water is correct, then even planets far from sources of water-rich asteroids may still hold water, and habitable worlds could germinate in a far wider range of circumstances and form faster than previously believed. It might even hold the keys to one day seeding our own habitable worlds beyond Earth.
It’s an interesting idea. So what do you think?